This invention is concerned with plungers for use in forming parisons during the manufacture of glass containers.
Two well-established processes for manufacturing glass containers are known respectively as the "press and blow" process and the "blow and blow" process and this invention is applicable to plungers for use in either of these processes. The plungers used are made of metal and have to withstand high compressive forces.
In the "press and blow" process, a gob of molten glass is introduced into a parison-forming or blank mould and a plunger is moved into the mould to press the glass to the shape of the mould cavity. The plunger is then withdrawn and the parison is transferred to a finish mould in which it is blown to the shape of the mould cavity by the introduction of air into the space previously occupied by the plunger. The plungers used in the "press and blow" process are relatively long (typically 100 mm) and, where narrow neck containers are being manufactured, may be small in diameter (e.g. 18 mm).
When light-weight containers are manufactured by the "press and blow" process, it is advantageous to use short parisons and allow time for the parison to stretch before it is blown in the finish mould. This stretching time, during which the heat contained in the parison is redistributed to bring the parison to a uniform temperature, limits production speeds and is a disadvantage of the "press and blow" process in contrast to the "blow and blow" process described below. The stretching time can be reduced to a minimum if the heat extracted from the glass by contact with the plunger can be limited and, it is believed, that a stronger container results if chilling of the inside of the parison can be reduced.
It is an object of the present invention to provide a plunger which, in the "press and blow" process, allows a reduction of the heat extracted from the glass by contact with the plunger.
In the "blow and blow" process the parison is formed by a blowing operation in a blank mould and the parison is transferred to a finish mould for a further blowing operation to form the parison into a container. In forming the parison, the gob is introduced into the mould on top of a short plunger which acts to form the neck region of the parison. Air is blown on top of the gob to "settle" the glass around the plunger. The plunger is then withdrawn creating a space into which air is blown to blow the parison to the shape of the mould cavity.
In the "blow and blow" process, the time during which the glass is "settled" by the application of air on top of the gob should be kept to a minimum in order to avoid a heavy "settle wave" in the completed container and also in the interests of productivity. It is desirable, therefore, to extract heat from the glass in the neck region as rapidly as possible. However the glass which contacts the tip of the plunger has to be softened, by re-heating from the remainder of the glass, before the parison can be blown and this causes a delay if too much heat has been extracted from the glass at that point.
It is a further object of the present invention to provide a plunger which, in the "blow and blow" process, allows low heat extraction in the tip region of the plunger and higher heat extraction in the side regions of the plunger.